This invention relates to an apparatus for detecting toner shortage in a developing unit used with printers and other image recording systems. More particularly, this invention relates to an apparatus for detecting toner shortage in a developing unit used with remote printers that record image data being sent continuously from a host computer.
An example of the printers used with a developing unit of the type contemplated by the present invention is a "remote printer" that records the image data being supplied continuously from a host computer on a round-the-clock basis. The remote printer is designed to perform recording operations in accordance with operational instructions supplied from the host computer and unlike ordinary copiers, the remote printer is almost always used without being attended by an operator. Hence, the developing unit used in such a remote printer has a lot of toner accommodated in a container so as to produce a large number of prints continuously. Further, the developing unit is equipped with a toner shortage detector that checks for presence or absence of the toner and that indicates toner shortage if there is no toner in the container.
The above conventional toner shortage detector, however, has the following problems. The toner shortage detector is so designed that when the toner is exhausted from the developing unit, the detector immediately gives an indication of toner shortage. However, as already mentioned, the remote printer associated with the developing unit is almost always used without being attended by an operator. Therefore, even if toner shortage is indicated immediately after the loss of toner from the developing unit, the printing operation may continue without the supply of an additional toner. If this occurs, prints having insufficient image density or those which have image skips can be produced in large quantities. Particularly in the case where the printing operation is continued until the toner is completely exhausted from the developing unit, image defects such as clear spots in prints cannot be completely eliminated even after an additional toner is supplied.
With a view to solving these problems, an apparatus for detecting toner shortage in the following manner may be used. When a toner shortage sensor detects the fact that the residual amount of toner in the developing unit has reached a predetermined level, the apparatus displays an instruction for supplying additional toner while the image recording operation is continued using the residual toner in the developing unit. The apparatus counts the number of prints produced after toner shortage is detected by the sensor, and stops the operation of the printer when a predetermined number of prints has been produced.
The operator of this apparatus supplies an additional toner into the container in the developing unit in response to the display indicating the need for toner replenishment and, then, he resets the counter for counting the number of prints to the initial value and resumes the recording operation of the printer. However, if the operator forgets to reset the counter after he supplies an additional toner, the counter will continue the counting operation, permitting the sensor to detect toner shortage in spite of the actual presence of the toner in the container. As a result, when the number of prints counted with the counter reaches a predetermined value, the printing operation of the printer stops. Since the operator may not become aware of this situation until a long time has passed, the "loss time" for which image data being sent from the host computer cannot be printed will be unduly extended.
This problem may be solved by automatically resetting the counter if the toner container is replenished with a toner, causing the toner sensor to detect the presence of the toner. In fact, however, there is no guarantee that the resetting operation is positively performed even if an additional toner is supplied into the container, since toner aggregation and other phenomena may prevent the toner sensor from immediately detecting the presence of the toner. If this occurs, the printer will again stop recording images when the number of prints being counted by the counter has reached a predetermined value. This problem can also occur in the case where a cleaning member is used to prevent toner deposition on the surface of the toner sensor for insuring its normal operation, because any contact between the cleaning member and the surface of the sensor causes the latter to detect the presence of the toner temporarily.
Further, the above apparatus for detecting toner shortage is so designed that a predetermined number of prints can be produced continuously even after the sensor has detected a toner shortage. Additionally the number of prints that can be produced is computed on the basis of image data having a standard image density. Therefore, if image data having a higher image density than the standard density is sent continuously from the host computer, the residual toner in the developing unit will be consumed before the predetermined number of prints are printed, potentially causing density drop or image skipping in the prints obtained.